Method for loading fibers contained in a pulp suspension

ABSTRACT

The present invention serves for loading a pulp suspension (S) with calcium carbonate. Liquid calcium hydroxide as milk of lime ( 7 ) is added during or before the pulp operation ( 1 ) carried out to form the pulp. By introducing gaseous carbon dioxide ( 8 ) into the pulp suspension thus treated, a chemical reaction is triggered therein, in which the finely divided calcium carbonate is precipitated. The present invention is particularly economic and effective due to the early addition of calcium oxide or calcium hydroxide.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of PCT application No. PCT/EP2006/004922,entitled “METHOD FOR LOADING FIBERS CONTAINED IN A PULP SUSPENSION”,filed May 24, 2006, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for loading fibers containedin a pulp suspension, and more particularly to a method of loadingcellulose fibers for producing paper or board.

2. Description of the Related Art

In the manufacture of paper, fillers, in particular precipitated calciumcarbonate (PCC) or crushed or ground calcium carbonate (GCC), arestandard substances which are used to reduce the fiber content and toimprove the optical properties of the paper.

Commercially available PCC or GCC fillers are products produced in bulkin special manufacturing facilities which can be associated with a papermill in the form of a satellite plant. The online production of PCC has,however, never been, nor is it now, considered in the paper industrybecause of the special process necessary to produce PCC. Instead, PCC orGCC is typically provided to the paper mills as bulk material or in theform of a suspension.

The loading with an additive, such as a filler can be effected, by wayof a chemical precipitation reaction, in particular by what is called a“Fiber Loading™” process, as described, inter alia, in U.S. Pat. No.5,223,090. In a “Fiber Loading™” process at least one additive, inparticular a filler, is incorporated onto the wetted surfaces of thefibrous material. In this process the fibers can be loaded with calciumcarbonate, calcium oxide and/or calcium hydroxide are added to the wet,disintegrated fibrous material in such a way that at least part thereofassociates with the water which is present in the fibrous material.Carbon dioxide is then added to the treated fibrous material. Uponaddition of the medium containing calcium oxide and/or calcium, whichhydroxide (preferably in liquid form (milk of lime)) to the pulpsuspension, a chemical reaction with exothermic properties takes place.This means that the water incorporated in or attached to the fibrousmaterial of the pulp suspension is not absolutely necessary for startingand running the chemical reaction.

An additional method for loading additive or filler is known from FR2831565 in which a suspension of mechanically produced, bleached woodpulp fibers (“pate a papier mecanique blanchie”) is loaded with calciumcarbonate. Therein, milk of lime is added to form the fiber suspensionand a consistency of above 10% is set. After diluting the suspension,the crystallization of calcium carbonate is triggered by the addition ofgaseous carbon dioxide.

An additional method for loading with filler is disclosed by WO03/066962. Therein, the result is further improved by a special grindingoperation.

Upon loading of the fibers with filler, calcium carbonate (CaCO₃) can beincorporated onto the wetted fiber surfaces by adding calcium oxide(CaO) and/or calcium hydroxide (Ca(OH)₂) to the moist fibrous material,at least a portion thereof being able to associate with the water of themass of fibrous material. Carbon dioxide (CO₂) can then be applied tothe fibrous material thus treated.

The term “wetted fiber surfaces” may cover all the wetted surfaces ofthe individual fibers. This also includes the case in which the fibersare loaded both on their outer surfaces and in their interiors (lumen)with calcium carbonate or any other desired precipitation product.

Accordingly, the fibers may be loaded with the filler calcium carbonate,the addition to the wetted fiber surfaces taking place by way of what iscalled a “Fiber Loading™” process, which is described as such in U.S.Pat. No. 5,223,090. In this “Fiber Loading™” process, the carbon dioxidereacts with the calcium hydroxide (in liquid or dry form) to form waterand calcium carbonate.

What is needed in the art is an economic method of processing fibers forthe production of a material web.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a method forloading fibers conformed in a pulp suspension, which is more economicthan the known methods.

Suitable starting materials with which the method can be performed arein principle any cellulose fibers suitable for producing paper or board.In one embodiment of the present invention, fiber loading can be carriedout with chemically produced pulp or with de-inked waste paper. Thelatter material, termed “dip”, similar to chemical pulp or waste paper,can be supplied to the paper mill in an air-dry state and then be pulpedand loaded in accordance with the present invention. With integrated dipproduction, printed waste paper is pulped, de-inked and loaded accordingto the present invention (see FIG. 2).

The present invention affords the possibility of adding the calciumoxide (CaO) and/or calcium hydroxide (CaOH₂) to the fibrous material ina state in which it is of relatively high consistency, e.g. air-dry ormoistened. Before introduction into the pulping apparatus and therein,the pulp is relatively absorbent, therefore, chemicals added thereinreach the wetted surface of the fibers more easily and quickly andpenetration into the cavities of the fibers takes place correspondinglymore quickly and more favorably. Since liquid, in particular water, isadded in the pulping apparatus, typically such as a pulper or a pulpingdrum, this process step still remains simple and clear. It is generallynecessary to meter in the calcium oxide or hydroxide. It is alsopossible to design the pulping apparatus such that the precipitation ofcalcium carbonate can already be triggered therein by adding at least aportion of the carbon dioxide to the apparatus. It may, for example, bedissolved in water. Alternatively, the pulping apparatus is temporarilyclosed and supplied with gaseous carbon dioxide. In such cases, asteeping section before the pulping apparatus is used in which thecalcium hydroxide (milk of lime) is sprayed on or is added in a steepingdrum.

A further advantage of the present invention is that under certaincircumstances it is possible to economize by omitting a machine forthickening, such as the disintegrated pulp during the puling operationis diluted only to such an extent that the subsequent loading can becarried out. In other cases, in particular should greater demands interms of cleaning of the stock be necessary (waste paper) or shouldseparation of the fibers not be adequately achieved otherwise, it isalso possible that thickening, such as in the form of a screw press, maybe necessary. The filtrate from such thickening is returned directlyinto the pulping apparatus in a short circuit, to serve for dilutiontherein. This has the decisive advantage that the calcium hydroxide orcalcium oxide is used optimally, as not only the water, but also thechemicals can be recirculated.

In many cases, the pulping operation is carried out not only with water,but also with the addition of pulping chemicals, in particular sodiumhydroxide solution, in order to provide a basic medium. Here too, thepresent invention has advantages since the sodium hydroxide solution canbe replaced by calcium hydroxide, which is required anyway for theprecipitation reaction resulting in calcium carbonate which is carriedout later.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is the method according to the invention in diagrammatic form;

FIG. 2 a special embodiment of the method for processing printed wastepaper; and

FIG. 3 a simplified plant diagram for performing the method according tothe invention.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrates one embodiment of the invention, in one form, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, whichillustrates an embodiment of the present invention, the pulp P, whichmay be waste paper or alternatively, fresh chemical pulp, is processedin pulping operation 1. Therein, a pulp suspension S is produced byadmixing water W with the aid of a mechanical working. The calciumhydroxide in this example is added as milk of lime 7 during the pulpingoperation, and is provided by slaking lime in the milk-of-limeproduction stage 5. Optionally, the pulp suspension S may be passedthrough a cleaning system, not shown here, in which heavy particles andplastics or other trash are removed. Whether this is necessary willdepend predominantly on the quality of the raw material. A thickeningoperation 2 may then take place, but is not always necessary and,therefore, is shown in broken lines. Filtrate 9 from thickeningoperation 2 is used for pulping operation 1. The precipitation 3 of finematter, in particular calcium carbonate, is triggered by addition ofgaseous carbon dioxide 8, which comes from a carbon dioxide productionstage 6.

Pulp suspension S′ thus processed can then be used for producing paper4, which generally requires a number of additional steps, not shownhere. In paper production stage 4, white water is produced, which issuited to being used again, in part, as returned water 9′ in pulpingoperation 1. Thus, not only is the water used again, but also anyremaining calcium hydroxide contained therein.

Now, additionally referring to FIG. 2, a pulp suspension S obtained bypulping 1 printed waste paper is treated by a flotation process 31 suchthat the ink particles and other fine anionic trash are removed asrejects 32. It is possible during the pulping operation 1 to replace thesodium hydroxide, which was hitherto conventionally used with calciumhydroxide in the pulping operation.

Now, additionally referring to FIG. 3 there is illustrated an example ofan installation with the most important process steps and devices. Inthis installation, the pulp P, together with water W, is introduced intoa pulper 10. This pulper 10 may be of conventional construction, i.e.contain a pulper screen 11 in its bottom region, and also a pulper rotor12 which circulates the pulp in the pulper and keeps the pulper screen11 free from clogging. The pulp is supplied to a plurality of cleaningdevices which are represented symbolically as a pulp cleaner 14 and apulp screener 15. There is also a screw press 16, in order to increasethe consistency of the pulp suspension. The press filtrate 18 from screwpress 16 can be returned for pulping in pulper 10. The thickened pulp 19then passes into a crystallizer 20, which serves, with the aid ofsupplied carbon dioxide gas 24, to trigger the desired precipitation ofcalcium carbonate in the form of finely-divided filler. The carbondioxide gas is provided by a carbon-dioxide supply device 21 and can bebrought to the desired temperature in a heat exchanger 22 with the aidof a cooling or heating device 23. There are, however, also otherpossible methods of temperature control, such as the direct addition ofsteam if it is expedient to increase the temperature. If necessary, aportion of the carbon dioxide gas 24′ may also be added aftercrystallizer 20 using a mixing device 25 in order to complete theloading operation.

The loaded pulp suspension is collected in a storage chest 26, possiblydiluted with water 27 and is then available as a pulp suspension 28 fora papermaking machine 29.

The part of the plant shown in FIG. 3 for loading with fillers, inparticular calcium carbonate, should be regarded only as a simplifiedexample. In many cases, a bleaching operation may also be integratedtherein. The present invention allows for the loading process then to beoperated as an online process in the paper mill.

In that case, at least one of the following apparatus and/or componentsmay be used for the online process: HC cleaner, static mixer, limeslaking apparatus, press, (in particular screw press or belt press),compensating reactor, crystallizer, a further static mixer, CO₂ supplydevice or additional CO₂ recovery device, optional CO₂ heater, optionaladdition of chemical bleaching agent, and a press-water tank.

The formation of crystalline precipitation particles results in theadvantage that higher gloss values can be achieved for the end product.

According to one embodiment of the present invention, the press filtrate9 is used, at least partially, as dilution water in crystallizer 20.

Mixing device 25 may be a static mixer and may be used in particular forfine adjustment of the pH value of the pulp suspension, preferablywithin a range of between 6 and 10, or between 6 and 8, or even between6.5 and 8.5.

A further embodiment of the present invention is that at least a portionof the required CO₂ is provided by a CO₂ recovery system. Thus, forexample, it can be recovered from flue gas from boilers or flue gas frompower stations.

Precipitation product particles of rhombohedral form, with a respectivecube size within a range from about 0.05 to about 2 μm may be produced.In certain cases it is also advantageous to produce precipitationparticles of scalenohedral form with a respective length within a rangefrom about 0.05 to about 2 μm and a respective diameter within a rangefrom about 0.01 to about 0.05 μm.

According to one embodiment of the present invention the solidsconcentration of the pulp suspension provided for precipitation is setin a range from about 5 to about 60% and preferably in a range fromabout 10 to about 35%.

According to one practical embodiment of the loading method, the carbondioxide of the pulp suspension is added at a temperature in a range fromabout −15 to about 120° C. and preferably in a range from about 20 toabout 90° C.

The paper manufactured may, therefore, contain fillers of a size ofabout 0.05 to about 5 μm, which increases the optical properties of theend product. The filler may be calcium carbonate, which occurs in naturein the form of calcite or calcspar, aragonite and, in the rarer form,vaterite. The filler may consist mainly of the form calcite, of whichover 300 different crystal forms are said to exist. The form of thefiller particles used may be, for example, rhombohedral with arespective cube size in a range from about 0.05 to about 2 μm orscalenohedral with a respective length in a range from about 0.05 toabout 2 μm and a respective diameter in a range from about 0.01 to about0.05 μm, depending on the type of paper to be manufactured in each case.

The filler is well distributed on and throughout the fibers, which meansthat no agglomeration of crystals in bundles is encountered. Therespective filler particle, namely the crystal, is provided individuallyspaced apart or separated on the fiber. The filler particle covers thefiber by attaching to the fiber, which improves the optical propertiesof the end product. The particle size is therefore essential forachieving optimal opacity. A high opacity is achieved when the colourspectrum of visible light is well scattered. If the colour spectrum isabsorbed, the colour black is produced. If the size of the fillerparticles drops below 0.2 to 0.5 μm, there is a tendency to transparencyand higher gloss.

To achieve good loading results, the process for producing the fillercrystals may be designed as follows and have the following variables:

-   -   moist pulp or pulp being disintegrated    -   calcium hydroxide in liquid or dry form    -   CO₂    -   gas zone    -   rotor (crystallizer)    -   stator (crystallizer)    -   production of crystals in a gas atmosphere without introduction        of mixing energy    -   mixing with low shearing    -   no pressure vessel

The pulp suspension mixed beforehand with Ca(OH)₂ is passed tocrystallizer 20, such as in a fluffer, refiner, disperger or the like ata consistency or solids concentration in a range from about 5 to about60%, preferably in a range from about 10 to about 35%. CO₂ is thensupplied to the pulp suspension. The CO₂ can be added at temperatures ina range between about −15 and about 120° C., preferably at temperaturesin a range between about 20 and about 90° C.

The pulp suspension passes into the gas zone of the crystallizer 20,where each individual fiber is exposed to a gas atmosphere, followed bythe precipitation reaction, which directly yields the CaCO₃. The form ofthe CaCO₃ crystals can be rhombohedral, scalenohedral or spherical. Theamount of crystals is dependent on the selected temperature range forthe pulp suspension and on the CO₂ and Ca(OH)₂ content in the pulpsuspension. Once the pulp suspension with the formed crystals has passedthrough the gas zone, the PCC formed or the pulp suspension with thecrystals in the lumen, on the fiber and between the fibers is passedthrough a rotor and a stator, where the distribution of the crystals inthe pulp suspension is concluded by mixing with low shearing.

While the pulp/crystal suspension passes through the rotor, a sheardistribution occurs which brings about a size distribution of thecrystals of about 0.05 to about 0.5 μm, preferably from about 0.3 toabout 2.5 μm.

The form of the filler particles used is rhombohedral with a respectivecube size in a range from about 0.05 to about 2 μm or scalenohedral witha respective length in a range from about 0.05 to about 2 μm and arespective diameter in a range from about 0.01 to about 0.5 μm,depending on the type of paper to be manufactured.

The further the pulp suspension has to meet the rotor disc, the less isthe shearing, depending on the H₂O added for dilution. The concentrationof the pulp suspension passing through the rotor disc is about 0.1 toabout 50%, preferably about 35 to about 50%.

The pressure acting on the CO₂ feed line is in a range from about 0.1 toabout 6 bar, preferably in a range from about 0.5 to about 3 bar, inorder to ensure a constant CO₂ supply to the gas ring for the desiredchemical reaction. As with supplying water via a garden hose, if thedemand for water is high the pressure should be increased to convey morethrough the hose. Since the CO₂ is a compressible gas, the necessaryamount can also be increased to ensure a complete reaction. The CO₂supply, and hence the precipitation reaction producing the CaCO₃ can becontrolled by open and/or closed-loop control by way of the pH value.

For example, pH values in a range from 6.0 to about 10.0 pH, preferablyin a range from about 7.0 to about 8.5 pH, can be considered for thefinal reaction of the CaCO₃ crystals. The energy used for this processcan lie in a range between about 0.3 and about 8 kWh/t, preferably in arange between about 0.5 and about 4 kWh/t. Dilution water can be addedand mixed with the pulp suspension in order to obtain a final dilutionin which the pulp suspension with filler which is produced has aconsistency or solids concentration in a range from for example about0.1 to about 16%, preferably in a range from about 2 to about 6%. Thepulp suspension is then exposed to the atmosphere in a machine, a tankor the next process machine.

The speed of rotation of the rotor disc can be in a range from about 20to 100 m/s, preferably in a range from about 40 to about 60 m/s at theexternal diameter. The gap between rotor and stator is, for example,about 0.5 to about 100 mm, preferably about 25 to about 75 mm. Thediameter of the rotor and of the stator can be in a range from about 5to about 2 m.

The reaction time is preferably in a range from about 0.001 to 1 min,preferably in a range from about 0.1 to about 10 sec.

The method described above permits the production of individualparticles which are spaced apart equally from each other and areattached to the fibers, covering the fibers in the necessary manner tomeet the requirements for the desired high degree on white or glosspaper. The particle size is preferably in a range from about 0.05 toabout 5 μm, the preferred size for the rhombohedral form of a cube beingin a range from about 0.05 to about 2 μm or for a scalenohedral formwith respect to the length in a range from about 0.05 to about 2 μm andwith respect to the diameter in a range from about 0.01 to about 0.5 μm.For high-gloss applications, the particle size should expediently bebelow 0.2 to 0.5 μm.

While this invention has been described with respect to at least oneembodiment, the present invention can be further modified within thespirit and scope of this disclosure. This application is thereforeintended to cover any variations, uses, or adaptations of the inventionusing its general principles. Further, this application is intended tocover such departures from the present disclosure as come within knownor customary practice in the art to which this invention pertains andwhich fall within the limits of the appended claims.

1. A method for loading a pulp suspension containing cellulose fiberswith calcium carbonate, comprising the steps of: introducing one of acalcium oxide and a calcium hydroxide in one of a liquid form and a dryform into a pulp; forming the pulp suspension from said pulp;introducing a gaseous carbon dioxide into the pulp suspension;precipitating calcium carbonate by way of the carbon dioxide; performinga pulping operation; and adding at least one of the calcium oxide andthe calcium hydroxide one of during said pulping operation and beforesaid pulping operation.
 2. The method of claim 1, wherein said pulpingoperation is carried out continuously.
 3. The method of claim 1, whereinsaid pulping operation is carried out discontinuously, at least aportion of the calcium hydroxide is metered into the pulp in a liquidform at a beginning of said pulping operation.
 4. The method of claim 1,wherein said pulping operation is carried out in a pulper.
 5. The methodof claim 1, wherein said pulping operation is carried out in a pulpingdrum.
 6. The method of claim 5, wherein at least one of the calciumoxide and the calcium hydroxide is added to a charging region of saidpulping drum.
 7. The method of claim 1, wherein said pulping operationis carried out in a consistency range of between 3% and 30%.
 8. Themethod of claim 7, wherein said consistency range is between 10% and20%.
 9. The method of claim 1, further comprising the step of steepingthe pulp with water and calcium hydroxide before said pulping operation.10. The method of claim 9, wherein said steeping step is carried outusing a steeping drum.
 11. The method of claim 9, wherein said steepingstep includes the step of spraying the calcium hydroxide onto the pulp.12. The method of claim 1, further comprising the step of adding thecalcium hydroxide in the form of milk of lime, said milk of lime havinga consistency in the range of 0.01% to 60% of existing solids content ofa dry paper mass.
 13. The method of claim 1, further comprising the stepof setting a time of between 0.01 and 10 minutes for the reacting of thecalcium hydroxide.
 14. The method of claim 1, wherein the pulp has aconsistency of between 40% and 100% of a starting material.
 15. Themethod of claim 1, further comprising the step of cleaning the pulpsuspension of anionic trash, said cleaning step being carried out aftersaid pulping operation.
 16. The method of claim 15, further comprisingthe step of executing a flotation process whereby ink particles areremoved to thereby clean the pulp suspension.
 17. The method of claim 1,further comprising the step of thickening the pulp suspension prior tosaid introducing a gaseous carbon dioxide step.
 18. The method of claim17, wherein a filtrate produced during said thickening step is used insaid pulping operation.
 19. The method of claim 1, further comprisingthe step of converting the pulp suspension into a fine fiber crumb in acrushing step one of during and after said precipitation step, the finefiber crumb being disperged.
 20. The method of claim 1, wherein saidprecipitation step is carried out at a consistency of between 5% and60%.
 21. The method of claim 20, wherein said consistency is between 10%and 35%.
 22. The method of claim 20, further comprising the step ofexpending energy executing said precipitation step, said energy beingbetween 0.3 and 8 kWh/t.
 23. The method of claim 22, wherein said energyis between 0.5 and 4 kWh/t.
 24. The method of claim 1, wherein saidprecipitation step is carried out at a temperature of between −15° and120° C.
 25. The method of claim 24, wherein said temperature is between20° and 90° C.
 26. The method of claim 1, wherein said precipitationstep is carried out at a pressure of between 0 and 15 bar.
 27. Themethod of claim 26, wherein said pressure is between 0 and 6 bar. 28.The method of claim 1, wherein said precipitation step is carried outwith a pH value of between 6 and
 10. 29. The method of claim 28, whereinsaid pH value is between 6.5 and 8.5.
 30. The method of claim 1, whereinsaid precipitation step is carried out within a reaction time of between0.001 minute and 1 minute.
 31. The method of claim 30, wherein saidprecipitation step is carried out within a reaction time of between 0.5second and 10 seconds.
 32. The method of claim 1, further comprising thestep of producing one of a paper and a board from the pulp suspensioncontaining loaded cellulose fibers, said producing step generating whitewater containing calcium hydroxide which is used as a returned water inone of said pulping operation and for diluting the pulp.
 33. The methodof claim 1, wherein the cellulose fibers used in said pulping operationconsist at least in part of de-inked waste paper.